The present invention describes improvements to the weld selector stations described in the above patent application. The improvements also are applicable to other electronic welders which use a plurality of paralleled transistors to provide the desired output current and/or which use different striking and welding voltages.
In welding and other power systems which provide a high current capability, a number of transistors are generally operated in parallel, with emitter resistors for current balancing, to provide the required output current. However, catastrophic failures, involving some or all of the output transistors, sometimes occur. This type of catastrophic failure usually results from an unusually large current surge which destroys first one transistor and then, by a chain reaction, causes the destruction of some or all of the remaining transistors. In the past, it was generally believed that the failure was caused by the large current destroying the emitter-base or the base-collector junction of the first transistor. However, it has been found that the failure of the first transistor is not always due primarily to the failure of the junctions but can be attributed to failure of the bonding wire between the emitter and the external connection to the power transistor. During large current surges, the transistor junctions are, to some degree, protected from failure by over heating because of the relatively large thermal mass of the junction and the high thermal conductivity between the junction, substrate, and heat sink. However, the emitter bonding wire has very little thermal mass and the thermal conductivity between the emitter bonding wire and the heat sink is very low. Therefore, the large current surge overheats the emitter bonding wire and causes a failure by causing the emitter bonding wire to melt through, detach from the emitter bonding pad, or detach from the emitter contact. The net result is that the emitter is suddenly opened, the V.sub.CBO rating of the transistor is exceeded and then the collector-base junction of the transistor breaks down. The failure of the collector-base junction of the first transistor applies the full collector voltage to the bases of the remaining transistors. This, in turn, prevents the remaining transistors from being turned off. These remaining transistors, in turn, draw large currents, overheat, and fail. Therefore, there is a need for a method of preventing current surges of such magnitude as to melt or detach the emitter bonding wire. Furthermore, there is a need for an apparatus to protect the other parallel transistors in the event that the collector-to-base junction of a first transistor fails.
Also, the voltage and current produced by the collector-base junction failure frequently overloads the driver circuit, thereby causing the driver circuit to be destroyed. There is therefore a need for a method of preventing destruction of the driver circuit in the event of a failure of one or more of the output transistors.
The weld selector station described in U.S. patent application Ser. No. 791,224 describes a peak current detector which, if the instantaneous current exceeds a predetermined value, interrupts the drive to the power transistors until the reason for the excessive current is corrected. However, in some situations, where the reason for the excessive current is correctable but persistent, the repeated periodic surges of the excessive current can cause overheating of the power transistors. Therefore, there is a need for an apparatus to protect the power transistors from overheating caused by repeated, excessive current flows.
Electronic welders, and many non-electronic welders, have a cooling fan to prevent the components of the welder from overheating in normal use. These fans generally have a DC motor since only DC power is available. These fan motors are generally conventional DC motors in that they have a pair of brushes, one for the positive terminal and one for the negative terminal. As these brushes wear, they must be periodically replaced or the fan will fail and the components of the welder will overheat. Generally, the brush connected to the positive terminal wears faster and, when it is replaced, there is frequently a substantial amount of brush left connected to the negative terminal. However, these brushes are generally replaced as a pair. It has been found that the time between replacement of the pairs of brushes can be extended if, periodically, the positive and negative terminals to the fan are reversed so that, alternately, a particular brush is connected to the positive voltage supply and then to the negative voltage supply. Therefore, there is a need for an apparatus for periodically reversing the polarity of the voltage applied to the fan motor.
The speed at which the fan operates also affects the lifetime of the brushes; high fan speeds mean shorter life and vice versa. Typically, the cooling fan has only one speed. However, the components of the welder generally require full cooling only when welding is actually being performed and can operate with reduced cooling between welding operations. Therefore, running the fan at high speed when welding operations are actually in progress and running the fan at a lower speed at other times will increase the lifetime of the brushes. Therefore, there is a need for an apparatus which will operate the cooling fan at high speed when welding operations are in progress and at a lower speed at other times.
A cooling fan failure will often go unnoticed by the welder because of the ambient noise level. However, when the cooling fan fails, the heat generated by welding operations and, in some cases, by a standby mode, can cause the components of the welding station to overheat and fail. Therefore, there is a need for an apparatus which automatically shuts down the welding station in the event that the cooling fan fails.
Even if the cooling fan is fully operational, if the air flow into the welding station is blocked or restricted, the ambient temperature is excessively high, and/or the particular welding operation causes an excessive amount of heat to be generated by the components of the welding station, the internal components of the welding station will overheat and fail. Therefore, there is a need for an apparatus which monitors the temperature inside the welding station and disables the welding station in the event that the internal temperature becomes excessively high.
During normal usage, the tip of the welding rod may become dirty or deformed, such as with a ball of metal on the end as opposed to a sharply truncated tip. In such cases, the striking and arc characteristics will be irregular until such time as the dirt or deformity is removed. It has been found that this dirt or deformity can be removed by applying a large current surge to the welding rod at the time the arc is first struck. This large current destroys the dirt or deformity at the end of the welding rod. Therefore, there is a need for an apparatus for applying a large current surge to the welding rod at the time of striking the arc so as to remove any dirt or deformity at the end of the welding rod.